The present invention relates to charged particle beam equipment such as an electron microscope and an ion microscope.
For observation in an electron microscope and an ion microscope, a specimen is usually magnified through a lens. In this case, the resolution of a magnified image is restricted by aberrations of the lens, which results in two orders of magnitude decrease in resolution as compared to the wavelength of a beam. To address this problem, an aberration correction technique to reduce the aberration of the lens has been developed. However, there is a problem of increased cost of the equipment.
A phase retrieval method or a Fourier iterative phase retrieval method (R. W. Gerchberg and W. O. Saxton, “A practical algorithm for the determination of phase from image and diffraction plane pictures”, Optik, vol. 35, pp. 237-246, 1972) is a method (hereinafter, referred to as “phase retrieval method” in this specification) for reconstructing an object image from a diffraction pattern.
A diffraction pattern can be obtained without using a lens, so that, in the phase retrieval method, it can be expected to observe an image with a high resolution and without the influence of the aberrations of the lens. In an X-ray technique where a lens can not be generally used, a technique for reproducing an object image from a diffraction pattern using the phase retrieval method has been advanced, which enables an image observation with a resolution of about the wavelength. Further, although a lens is used, an example of applying the phase retrieval method to an electron microscope has been reported (J. M. Zuo et al., “Atomic Resolution Imaging of a Carbon Nanotube from Diffraction Intensities”, Science, Vol. 300, pp. 1419-1421, 2003).